Can someone please explain to me why text messages which have not been delivered to a phone expire after 72 hours? I went on holiday for 10 days and was out of range. When I got back into range I did not get any of the messages that had been sent more than 3 days before and some of the ones sent more recently too.Help!

Because messages which are not delivered sit in a queue which is retried on a regular basis (every 4-8 hours) until it is delivered or it expires. If they did not get expired the network and the SMSC would be swamped by retries of messages which were never going to get delivered (lost/unused phones, broken SIMs, etc). Even with expiry 50% or more of traffic from the SMSC can be retries.